Oil is a nonrenewable natural resource having great importance to the industrialized world. The increased demand for and decreasing supplies of conventional oil has led to the development of alternate sources of oil such as deposits of bitumen and heavy crude as well as a search for more efficient methods for recovering and processing hydrocarbons extracted from such deposits.
There are substantial deposits of oil sands in the world, with particularly large deposits in Canada and Venezuela. For example, the Athabasca oil sands region of the Western Canadian Sedimentary Basin contains an estimated 1.3 trillion barrels of potentially recoverable bitumen. An equally large deposit of bitumen may be found in the Carbonates of Alberta. There are lesser, but significant deposits, found in the U.S. and other countries. These oil sands and carbonate reservoirs contain a petroleum substance called bitumen or heavy oil. Bitumen deposits cannot be economically exploited by traditional oil well technology because the bitumen or heavy oil is too viscous to flow at natural reservoir temperatures.
When oil sand deposits are near the surface, they can be economically recovered by surface mining methods. The current principal method of bitumen recovery, for example, in the Alberta oil sands is by conventional surface mining of shallower deposits using large power shovels and trucks to feed a nearby slurry conversion facility which is connected to a primary bitumen extraction facility by a long hydrotransport haulage system. The bitumen is finally taken to an upgrader facility where it is refined and converted into crude oil and other petroleum products.
When oil sand deposits are too far below the surface for economic recovery by surface mining, bitumen can be economically recovered in many, but not all, areas by recently developed in-situ recovery methods, such as Steam Assisted Gravity Drain (“SAGD”), VAPEX, and other variants of gravity drainage technology to mobilize the bitumen or heavy oil. The principal method currently being implemented on a large scale is SAGD. Typically, SAGD wells, or well pairs, are drilled from the earth's surface down to the bottom of the oil sand deposit and then horizontally along the bottom of the deposit. The wells inject steam to reduce the viscosity of bitumen. The wells then collect the mobilized bitumen.
Heat Assisted Gravity Drain (“HAGD”) is a relatively new process for mobilizing bitumen in the Alberta oil sands and in carbonates. Electric heater elements are embedded in the reservoir material and used, in place of steam, to heat the formation until the bitumen becomes fluid enough to flow by gravity drain. HAGD may require more energy than SAGD but may be used in reservoirs where SAGD cannot—such as, for example, reservoirs with poor steam caps. HAGD and SAGD may also be used in combination, where HAGD elements are used to melt the bitumen around the steam injectors, thereby allowing the steam chamber to form more quickly. Solvents such as carbon dioxide and paraffins and/or olefins may also be used to help mobilize the bitumen or heavy oil such as for example in SAGD/HAGD and other solvent recovery operations or combinations thereof.
Because of global warming concerns, this potential for substantially increasing carbon dioxide emissions may outweigh the advantages of the enormous reserves of unconventional hydrocarbon deposits available.
Even the most efficient SAGD or HAGD operation requires substantial amounts of energy to deliver the required amount of steam or heat to the reservoir to mobilize the bitumen. If this energy is obtained by burning fossil fuels, there is the potential to generate significant amounts of carbon dioxide emissions during recovery operations. The thermal energy required to mobilize bitumen can be quantified by a Steam-Oil-Ratio (“SOR”), which is determined by the number of barrels of water required to produce the steam divided by the number of barrels of oil or bitumen recovered. In a SAGD operation having an average SOR of 3, the energy required to produce high quality steam to recover 1 barrel of heavy oil or bitumen oil is equivalent to about ¼ of a barrel of oil. Thus, oil produced by thermal recovery methods have the potential to generate 25% or more carbon dioxide emissions than oil recovered by pumping from conventional oil wells.
In addition, the upgrading process when carried out underground, such as described for example in U.S. Pat. No. 7,066,254 or at a surface refinery can generate additional carbon dioxide and other unwanted emissions.
There has been much effort to utilize all the on-site water and energy potential derived from a SAGD operation to increase the overall efficiency of the operation and to prepare the produced bitumen or heavy oil for pipeline transmission over existing pipeline networks.
There remains, therefore, a need for a method to eliminate carbon dioxide emissions generated during thermal recovery operations of unconventional oil such as heavy oils and bitumen. Further, there is a need to utilize any number of readily available fossil fuels to power thermal recovery operations while substantially reducing or eliminating carbon dioxide emissions.